1. Field of the Invention
The present invention relates to a channel response estimation apparatus for estimating a channel response in an OFDM radio communication system, and a receiving device using the channel response estimation apparatus.
2. Description of the Related Art
In high speed radio communication systems, compensation for signal waveform distortion due to multipath delay waves has become indispensable. As a modulation scheme for use in radio communication, orthogonal frequency division multiplexing (OFDM) is known which utilizes a plurality of carriers and each carrier is orthogonal to others (called sub-carriers).
In OFDM, inter-symbol interference can be avoided by converting an OFDM signal into a cyclic signal using guard intervals. This prevents signal waveform distortion due to multipath delay waves, without an equalizer. In an environment in which signal waveform distortion occurs because of multipath delay waves, subcarriers of an OFDM signal are received with different gains and phases. To demodulate OFDM signal in such environment, it is necessary to estimate the frequency transfer function of the channel and perform demodulation based on the estimation result.
In OFDM, in general, a predetermined known signal, which is already known to a transmission side and reception side, hereinafter called “known signal” is transmitted to estimate the frequency transfer function of a channel. In a method in which known signals are carried by all subcarriers in each OFDM symbol, it is not necessary to perform frequency-domain interpolation, which is effective when the range of variations with time in the response of the channel is small. Further, a scheme is well known in which the impulse response of a channel is estimated from such a known signal, and the estimated impulse response is subjected to Fourier transform, thereby estimating the frequency transfer function of the channel.
Estimation of an impulse response is realized by, for example, modeling a channel using a transversal filter model, and estimating the tap coefficient of the filter. Jpn. Pat. Appln. KOKAI Publication No. 2003-124857 (document 1) discloses a method for estimating an impulse response using the least-square method. Jpn. Pat. Appln. KOKAI Publication No. 2003-32217 (document 2) discloses a method for estimating an impulse response using the mean square error method.
In OFDM, in general, to reduce the interference between adjacent channels, several subcarriers located at the both edges of whole frequency band of an OFDM signal cannot be used for communication. This degrades the time-domain resolution. Accordingly, the method proposed by document 1 cannot separate delay waves that reach adjacent taps. In the method proposed by document 2, the estimation performance is significantly influenced by the initial value of the reverse matrix of a correlation matrix used in the recursive least square (RLS) method.